A Molecular Theory of Disease

The atom is the basic building block of matter. The model of the atom is a positively charged nucleus and surrounding negative electrons. The chemical behavior of an atom depends on the numbers of these electrons. Most atoms exist as part of molecules. All life on earth depends on molecular processes. Most of these processes need a molecular catalyst to speed up chemical reactions taking place at body temperature. 

Molecules are the smallest parts of matter that retain the chemical properties of the substance. A molecule consists of at least two atoms held together by chemical bonds. Everything in the body is made of molecules. If the nucleus of an atom is very large, the atom is often unstable, undergoes radioactive decay, and emits smaller particles or high-energy photons. With special instruments, these particles can be detected from outside the body. 

An example of a molecular disease is the panic state, which results from the flooding of the brain with epinephrine (adrenaline), a hormone carried in the blood. It is also a neurotransmitter released into neuronal synapses of activated neurons of the autonomic nervous system. Both epinephrine and norepinephrine are responsible for the emotions anger and fear. 

Damage to the blood-brain barrier (BBB) as a result of trauma or disease permits hormones (molecules), such as epinephrine, or toxic substances to reach the brain. Normally, the BBB keeps the contents within blood vessels from reaching the brain. Closely packed endothelial cells line blood vessels. The endothelial cell membrane has transport systems that facilitate the movement of desired molecules and nutrients into the brain, while keeping undesirable molecules out. Damage to the BBB from trauma may be involved in posttraumatic stress disorder. 

Epinephrine is released from the adrenal medulla by activation of the sympathetic nerves of the autonomic nervous system. Norepinephrine is released from the adrenal medulla. About 20% of the total catecholamine released from the adrenal is norepinephrine. Most norepinephrine released by sympathetic nerves is taken back up into presyn-aptic neurons. A small amount diffuses into the blood and circulates throughout the body. When the sympathetic nervous system is highly activated, the amount of norepinephrine entering the circulation increases. 

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